In September 1698, the most mysterious prisoner in French history, known as the "The iron mask", was transferred to the Paris Bastille.
He had been in prison for 30 years, had changed many prisons and was always wearing a mask. According to some sources, in velvet, according to others in iron. He was richly dressed, played the violin in the cell, he was well fed.
There are many versions of historians about the identity of this man: an illegitimate son of the king or a minister who stole, a half-brother or even a twin of Louis XIV himself, who was imprisoned in prison to avoid conflicts with the succession to the throne. 
It was this hypothesis that formed the basis of Alexandre Dumas' novel "The Vicomte of Bragelonne: Ten Years Later", thanks to which the "iron mask" became a character in many books and films.
It is known for certain that he died in the Bastille in 1703.
_Images: History Today and _National Geographic